In the graphic arts, it is desirable to produce a color proof to assist a printer in correcting a set of photomasks which will be used in exposing printing plates. The proof should reproduce the color quality that will be obtained during the printing process. The proof must be a consistent duplicate of the desired half tone or line image, and should neither gain nor lose color. Visual examination of a color proof should reveal the following characteristics:
1. Any defects on the photomask.
2. The best color rendition to be expected from press printing of the material.
3. The correct gradation of all colors and whether grays are neutral.
4. The need, if any, for subduing any of the colors and/or giving directions for altering the film photomask before making the printing plates.
Color proofing sheets for multi-colored printing have heretofore been made by using a printing press proof which requires taking all the steps necessary for actual multicolor printing. Such a conventional method of color proofing has been costly and time consuming. Alternate color proofing methods have therefore been developed to simulate the quality of press proofs. There are two known types of photographic color proofing methods, namely, the surprint type and the overlay type.
In the overlay type of color proofing, an independent transparent plastic support is used for producing an image of each color separation film. A number of such supports carrying colored images are then superimposed upon each other and placed on a white sheet to produce a color proof. The overlay type of color proofing method has the disadvantage that the superimposed plastic supports tend to darken the color proofing sheet, and, as a result, the impression of the color proofing sheet thus prepared becomes vastly different from copies actually obtained by a conventional printing press proof. Its primary advantage is that it is quick and can serve as a progressive proof by combining any two or more colors in register.
In the surprint type of color proofing method, a color proofing sheet is prepared by successively producing images of different colors from different color separation films onto a single receptor sheet. This is done by utilizing a single opaque support and by applying toners, photosensitive solutions or coatings of photosensitive materials of corresponding colors on the opaque support in succession. An example of this approach is described in U.S. Pat. No. 3,671,236. An advantage of the surprint type of color proof is that the color saturation is not influenced by superimposed plastic supports. This method more closely resembles the actual printing and eliminates the color distortion inherent in the overlay system.
Various processes for producing copies of an image embodying photopolymerization and thermal transfer techniques are known as shown in U.S. Pat. Nos. 3,060,023; 3,060,024; 3,060,025; 3,481,736; and 3,607,264. In these processes, a photopolymerizable layer coated on a suitable support is imagewise exposed to a photographic transparency. The surface of the exposed layer is then pressed into contact with the image receptive surface of a separate element and at least one of the elements is heated to a temperature above the transfer temperature of the unexposed portions of the layer. The two elements are then separated, whereby the thermally transferrable, unexposed, image areas of the composite transfer to the image receptive element. If the element is not precolored, the tacky unexposed image may now be selectively colored with a desired toner. The colored matter preferentially adheres to the clear unpolymerized material. U.S. Pat. No. 3,574,049 provides a transfer process for printing a design on a final support which comprises (a) printing a design onto a temporary support, (b) superimposing the temporary support and the final support, (c) applying heat and/or pressure to the superimposed structure formed in (b), and (d) separating the temporary support from the final support which retains the printed design. The affinity of the design for the temporary support is lower than its affinity for the final support.
In U.S. Pat. No. 3,721,557 a method of transferring colored images is claimed which provides a stripping layer coated between the photosensitive element and the support. When the photosensitive layer is exposed to actinic light and developed, the more soluble portions are selectively removed to produce a visible image. The image-carrying support is pressed against a suitable adhesive coated receptor and, subsequently, the carrier support sheet is stripped to accomplish the transfer of the image. A fresh layer of adhesive is applied to the receptor for each subsequent transfer.
U.S. Pat. Nos. 4,260,673 and 4,093,464 describe positive working one-piece proofing systems based on orthoquinone diazides. In U.S. Pat. 4,093,464 a colored image is transferred to a receiver sheet after exposure and development. U.S. Pat. 4,260,673 describes transfer of a solid color layer to a receiver sheet prior to exposure and development. U.S. Pat. No. 4,659,642 teaches a positive working color proofing system which has a transparent substrate, a colored photosensitive layer on the substrate, and a top adhesive layer. The present invention improves upon the foregoing by incorporating an improved positive working color proofing film comprising a poly(vinyl acetal/vinyl alcohol/vinyl acetate) terpolymer in the photosensitive layer.